1. Field of the Invention
The present invention relates generally to optical amplifiers used in fiber optics for telecommunications. More particularly, the invention relates to a Raman optical fiber amplifier and method and apparatus for enabling dynamic self-adjusting gain optimization and for equalizing amplified optical output.
2. Background Art
In optical fiber communication systems, communication channels can be provided by transmitting signals impressed on laser beams having different wavelengths (WDM). Although optical fiber communication systems utilizing wavelength-distinct modulated channels may carry information over long distances, signals transmitted through optical fibers are attenuated mainly by the cumulative and combined effects of absorption and scattering. While the signal attenuation per kilometer in optical fibers used for communications is typically low, signals transmitted over increasing transmission distances require periodic amplification over long distances. Amplification in fiber optic communication systems is performed mainly by electronic repeaters, Erbium doped fiber amplifiers (EDFA""s), semiconductor optical amplifiers, waveguide amplifiers and Raman amplifiers.
While amplification using electronic repeaters involves optical to electrical to optical conversions, amplification using EDFA""s, semiconductor optical amplifiers. waveguide amplifiers and Raman amplifiers is performed directly on optical signals, involving no optical to electrical to optical conversions. The Raman amplification process significantly differs from other amplification methods mentioned, as the transmission line itself can serve as the gain medium, whereas a module or component dedicated for the amplification process is used in other amplification methods. The Raman amplification process is based on the Raman effect, which describes conversion or scattering of a fraction of the optical power from an incident optical beam having a higher optical frequency to an optical beam having a lower optical frequency. The optical frequency shift between the incident beam and the scattered beam is determined by the vibrational states of the medium through which both beams are propagating. The Raman effect in silica-based fibers is described by quantum mechanics as scattering of an incident photon by a molecule to a photon with a lower optical frequency, while the molecule makes a transition between two vibrational states of the medium. Raman amplification involves Stimulated Raman scattering, where the incident beam, having the higher optical frequency, often referred to as the pump beam, is used to amplify the lower optical frequency beam, often referred to as the Stokes beam or the signal beam, through the Raman effect. The pump beam pumps the molecules of medium to an excited vibrational state, while the photons of the signal beam propagating through the excited molecules stimulate the emission of photons at the signal frequency, thereby amplifying the signal while the excited molecules return to their lower vibrational states [See for example xe2x80x9cNonlinear fiber opticsxe2x80x9d by G. P. Agrawal, pp. 316-369, Academic Press, 2nd edition, 1995]. Stimulated Raman scattering may involve a multiplicity of pumps at different frequencies and a multiplicity of signals at different frequencies.
A Raman amplifier, which is based on the Stimulated Raman scattering effect, may amplify a single optical channel, as well as collectively amplify a series of optical signals, each carried on a wavelength corresponding to a distinct channel. A Raman amplifier with a single pump source, where a fiber optic is used as the gain medium can amplify signals extending over a wide frequency range, referred to as the Raman gain spectrum or the Raman gain band. The Raman gain spectrum in silica optical fibers extends over a wide frequency range, with a broad peak downshifted by about 13 THz from the pump frequency. The Raman gain spectrum in optical fibers is not associated with fixed energy levels of the gain medium, as with rare earth element dopants in glass based fibers such as Erbium. Consequently, Raman amplification can be achieved practically at any wavelength in the near infra-red spectrum, as long as the appropriate pumping light source is available. This advantage allows Raman amplification to be applied for optical communications across the entire optical communication transmission window of silica optical fibers.
Raman amplification in optical fibers was thoroughly investigated in the seventies [R. G. Smith, xe2x80x9cOptical power handling capacity of low loss optical fibers as determined by Stimulated Raman and Brillouin scatteringxe2x80x9d, Applied Optics, Vol. 11 No. 11, p. 2489, 1972, R. H. Stolen et al., xe2x80x9cRaman gain in glass optical waveguidesxe2x80x9d, Applied Physics Letters, Vol. 22 No.6, p. 276, 1973, and J. Auyeung et al., xe2x80x9cSpontaneous and stimulated Raman scattering in long low loss fibersxe2x80x9d, Journal of Quantum Electronics, Vol. QE-14 No. 5, p. 347, 1978]. By the early eighties, the use of Raman amplifiers in optical communication systems had been proposed for multi-wavelength transmission [Mochizuki et al., xe2x80x9cOptical repeater system for optical communicationxe2x80x9d, U.S. Pat. No. 4,401,364; Hicks, Jr. et al., xe2x80x9cOptical communication system using Raman repeaters and components thereforxe2x80x9d, U.S. Pat. No. 4,616,898; and Mollenauer et al., xe2x80x9cOptical communications system comprising Raman amplification meansxe2x80x9d, U.S. Pat. No. 4,699,452]. However, reliable commercial and affordable high power means for Raman pumping of single mode fibers did not exist in the 1980s, and Raman amplification was usually considered for highly special uses such as Soliton transmission [Mollenauer et al., xe2x80x9cOptical communications system comprising Raman amplification meansxe2x80x9d, U.S. Pat. No. 4,699,452].
In the late 90""s, as high power EDFA""s became common, reliable high power pump laser diodes at the 1480 nm wavelength range were commercially available. As this wavelength range is also suitable for pumping of silica fibers Raman amplifiers [See for example xe2x80x9cErbium-Doped Fiber Amplifiersxe2x80x94Fundamentals and Technologyxe2x80x9d, by P. C. Becker et al., pp. 346-351, Academic Press, 1999], Raman amplifiers received renewed attention [Grubb et al., xe2x80x9cArticle comprising a counter-pumped optical fiber Raman amplifierxe2x80x9d, U.S. Pat. No. 5,623,508; Grubb et al., xe2x80x9cArticle comprising low noise optical fiber Raman amplifierxe2x80x9d, U.S. Pat. No. 5,673,280; Kerfoot et al., xe2x80x9cLightwave transmission system employing Raman and rare-earth doped fiber amplificationxe2x80x9d, U.S. Pat. No. 6,038,356; Kidorf et al., xe2x80x9cWide bandwidth Raman amplifier capable of employing pump energy spectrally overlapping the signalxe2x80x9d, U.S. Pat. No. 6,052,219; Kidorf et al., xe2x80x9cOptical fiber communication system with a distributed Raman amplifier and a remotely pumped Er-doped fiber amplifierxe2x80x9d, U.S. Pat. No. 6,081,366; Y. Akasaka et al., xe2x80x9cRaman amplifier optical repeater and Raman amplification methodxe2x80x9d, WO005622A1; T. Terahara et al., xe2x80x9c128xc3x9710 Gbits/s transmission over 840-km standard SMF with 140-km optical repeater spacing (30.4-dB loss) employing dual-band distributed Raman amplificationxe2x80x9d, paper PD28, Optical Fiber Communication Conference 2000, Baltimore, Md., USA, Mar. 7-10, 2000]. In comparison with EDFAs, Raman amplifiers allow amplification of a wider optical spectrum, with typically lower optical noise and over longer distances.
In contrast to EDFAs, where amplification properties are dependent only on the EDFA module, the transmission line itself can be used as the gain medium of a Raman amplifier, and thus, amplification properties such as gain and gain equalization are closely related to the type, properties and characteristics of the fiber used and the fiber condition (e.g. the distribution of losses along the fiber, the fiber effective area, Raman gain coefficient of the fiber and fiber length) [C. Fludger et. al., xe2x80x9cAn analysis of the improvements in OSNR from distributed Raman amplifiers using modern transmission fibersxe2x80x9d, paper FF-2 Optical Fiber Communication Conference 2000, Baltimore, Md., USA, Mar, 7-10, 2000.]. Thus it is impossible to accurately predict the performance of the Raman amplifier, including gain, gain equalization and noise properties, without a thorough knowledge of the fiber types properties, characteristics and conditions along the fiber optic transmission line. Moreover, the condition of the transmission line can have another critical influence on Raman amplifier performance. If the physical contact between two connectors joining two segments of the line is inadequate (such as air gaps between connectors), an electric arc at that point can be caused by the high optical power density associated with the Raman pumping. Such an electric arc may cause high losses at the point where it occurs, and can render the whole transmission line unusable [Jander et al., xe2x80x9cInterlocked high power fiber system using OTDRxe2x80x9d, U.S. Pat. No. 5,966,206].
As result of the dependence of Raman amplifier performance on the optical fiber characteristics (along the first tens of kilometers from the pump source) there is a need for a Raman amplifier that incorporates a transmission line diagnostic mechanism in order to calculate, adjust and optimize in-situ the operating parameters of the amplifier. Because the transmission line characteristics, as well as the transmitted signals power can change in time (e.g. fiber degradation due to aging and maintenance), this mechanism has to be able to continue performing tests on the optical fiber transmission line during the operation of the Raman amplifier.
There is also a need for this diagnostic mechanism to be able to prevent initiation or continuation of Raman amplification, for example, when launching high power pump into the transmission line can render the whole transmission line unusable.
This invention describes an apparatus and method for a Raman amplifier for fiber optic transmission lines incorporating a line diagnostic mechanism in order to test and characterize the line, calculate, control and optimize the operating parameters of the amplifier. This need arises because the amplifier gain medium is the transmission line itself and the amplifier properties depend on the optical fiber properties along the first tens of kilometers from the pump source of the transmission line.
According to the present invention there is provided a method of operating an optical fiber transmission line wherethrough signals are transmitted in a signal wavelength band, including the steps of: (a) pumping the transmission line, using at least one optical pump, thereby effecting Raman amplification of the signals; (b) determining at least one characteristic of the optical fiber transmission line; and (c) adjusting a power of the at least one optical pump in accordance with the at least one characteristic.
According to the present invention there is provided an amplifier for an optical fiber transmission line, including: (a) at least one optical pump for pumping the transmission line to amplify signals that are transmitted therethrough by Raman amplification; (b) at least one monitoring system for determining at least one characteristic of the transmission line; and (c) a line analyzing unit for adjusting a power of the at least one optical pump in accordance with the at least one characteristic.
According to the present invention there is provided a method of operating an optical fiber transmission line wherethrough signals are transmitted, including the steps of: (a) pumping the transmission line, using at least one optical pump, thereby effecting Raman amplification of the signals; (b) measuring a power of the signals; and (c) adjusting a power of the at least one optical pump in accordance with the power of the signals.
According to the present invention there is provided an amplifier for an optical fiber transmission line, including: (a) at least one optical pump for pumping the transmission line to amplify signals that are transmitted therethrough by Raman amplification; (b) a monitoring system for measuring a power of the signals; and (c) a line analyzing unit for adjusting a power of the at least one optical pump in accordance with the measured power of the signals.
A Raman optical fiber amplifier for communications is composed of a single or several high power light sources, typically high power laser diodes that act as optical pumps, typically at wavelengths between 1400 and 1500 nm. The amplification is generally performed along the optical fiber transmission line itself, by a non-linear Raman processes. A line analyzing unit, adjacent to the Raman pump unit, determines the types of optical fibers installed along the transmission line, the optical gain and loss distribution along them and other properties. The line analyzing unit may also determine if there exists optical loss that can be destructive when the high power Raman pump light is traversing through the line.
Data received after performing the tests by the line analyzing unit, allows the line analyzing unit to perform calculations required for optimization of the gain, gain equalization and gain tilt for the Raman amplifier, and to enable or disable the Raman pump or pumps. This information can be delivered either to the amplification management unit, or stored in the unit performing the Raman pumping. These tests can be performed continuously during the course of Raman pumping, and can be used to change operational conditions and status when changes along the transmission line occur.
The invention described herein significantly improves upon the prior art by providing an optical fiber transmission line diagnostic mechanism in order to calculate, adjust and optimize in-situ the operating parameters such as the optical power of the pump source or sources of the Raman amplifier. This diagnostic mechanism can also initiate shut-down of the high power pumps, and also prevent initiation of Raman amplification when breaks or cracks are present in the transmission line that would prove destructive. The invention has the capability to continuously monitor the optical fiber transmission line by performing tests during the operation of the Raman amplifier.